One example of a radio-frequency communications link for use in connecting data terminal equipment (DTE) is an asynchronous data service proposed for the INMARSAT-B.TM. or INMARSAT-M.TM. satellite communications system, as described for example in Chapters 12 and 14 of "Satellite Communications: Principles and Applications" by Calcutt and Tetley, 1st edition, published by Edward Arnold.
The overall layout of the satellite communications system, when used for data communications, is shown in FIG. 1. A mobile DTE 2 is connected via an RS232C interface to a modem interface unit (MIU) 4. The MIU 4 simulates a Hayes-compatible modem and is able to decode Hayes-type commands from the mobile DTE 2, so that off-the-shelf communications software may be used in the mobile DTE 2. The MIU 4 does not perform modulation or demodulation in this case, since it is not connected to an analog line. Instead, the MIU 4 provides an interface to a mobile earth station (MES) 6 which allows communication via a satellite 8 to a fixed or land earth station (LES) 10. The LES 10 is connected to an LES MIU 12 which interfaces the satellite link to a public switched telephone network (PSTN) 14 and therefore functions as a modem to convert analog signals on the PSTN 14 to digital signals on the satellite link, and vice versa. A fixed DTE 18 is connected to the PSTN 14 through a modem 16 of standard type. Alternatively, the LES MIU 12 may be connected to the fixed DTE via an ISDN and an ISDN adapter, or via another type of network allowing data to be sent in another format.
FIG. 2 shows the MES MIU 4 and the MES 6 in greater detail. The MES MIU 4 comprises a DTE interface 20, which provides an RS232 physical interface and emulates an AT.PCCA type modem, i.e. it complies with the minimum functional specification for data transmission systems published by the Portable Computer and Communications Association (PCCA), including the use of the AT command set and responses.
Data received by the DTE interface 20 is sent to a buffer 22, which is in turn connected to an MES interface 24. The MES interface 24 implements, in ARQ (automatic repeat request) mode, a variant of the HDLC (High Level Data Link Control) protocol, as defined in ISO recommendations ISO/IEC 3309, ISO/IEC 4335: 1993 and ISO/IEC 7809: 1993. The particular version employed is ISO HDLC BAC 3.2, 4, 8, 10, 12 as defined in ISO 7809: 1993 (synchronous, two-way simultaneous, duplex, non-switched). A controller 26 controls the operation of the interfaces 20 and 24 and the flow of data through the buffer 22.
The MES includes an RF modulator/demodulator 27, connected to an antenna 28, for RF modulating the output of the MES interface 24 and transmitting the output through the antenna 28 to the satellite 8, and for RF demodulating RF signals received from the satellite 8 through the antenna 28 and sending the demodulated signals to the MES interface 24. The MES 6 also includes access control and signalling equipment (ACSE) 30, for setting up and clearing the satellite link, which exchanges data with the controller 26 of the mobile MIU 4.
The MES ACSE 30 communicates with a network control station (NCS) which allocates communications channels and supervises communications traffic through the satellite 8 and communicates with further ACSE at the LES.
The mobile MIU 4, MES 6 and ACSE 30 may be integrated in a mobile unit and the antenna 28 may be integrated or connected externally with the mobile unit.
FIG. 3 shows the LES 10 and the LES MIU 12 in greater detail. The LES MIU 12 includes a modem 31 for demodulating analog signals from the PSTN 14 and modulating digital signals for the PSTN 14, and a modem interface 32 which supports modem protocols such as V.42 error correction, for communication with the modem 16. If the PSTN 14 is a digital network, a suitable interface is used instead of the modem 30.
The modem interface 32 is connected through a buffer 34 to an LES interface 36, which implements protocols compatible with the MES interface 24, so that data can be exchanged between the LES MIU 12 and the MES MIU 4. A controller 38 supervises the operation of the modem interface 32, buffer 34 and LES interface 36. The LES interface 36 is connected to an RF modulator/demodulator 40 which modulates signals for transmission to the satellite 8 through an antenna 42, and demodulates signals received from the satellite 8 though the antenna 42. Call set-up and clearing are controlled by an LES ACSE 44 within the LES 10 which exchanges signals with the LES MIU 12, the MES ACSE 30, and the network control station (NCS).